What are some exercises I can do at 350 pounds
If you have access to a pool water exercises are great, but I know many of us don't have that option.
Youtube has a bunch of videos for seated exercises for people with limited mobility something like that might be an option. Resistance bands are a good choice also.
Any walking is better than none. If all you can do it to the end of the driveway and back at least you did something. When you build up your tolerance go a little further. Also look for some chair exercises on YouTube or TikTok. Resistance bands and light weights are great for muscle building, but you need to get in your cardio as well.

HW 299 SW 290 CW 139 GW 140 2/08/2019 OPERATION: Surgical Hernia with excision of total surface area of 55 x 29 cm of abdominal skin.
Starting to move your body can feel challenging, but even small steps can make a big difference. If you are not healthy enough for high impact aerobics start with low intensity and work your way up. Walking, swimming, chair exercises or water aerobics are all great places to start.
As far as strength goes, find a good trainer that can scale exercises to your needs and help you focus on proper form from the start. A good trainer will be able to progress you from the beginning stages of working out to being strong enough to do more traditional gym movements once you have progressed.
One thing you want to pay close attention to is posture and proper form. If you start doing things incorrectly you run a very high risk of injury. This can be hard for some people as it takes time to learn this skill. A lot of trainers offer the first session free so find one and get started.
I hope this helps! Good luck